Production of type IV collagen is essential for the growth of normal mammary epithelium and mammary adenocarcinomas since selectively blocking the production of this protein causes growth arrest. Collagen production by adenocarcinomas is apparently constituitive because they produce factors that are potent stimulators of collagen synthesis. Mammary carcinomas which do not synthesize type IV collagen also do not produce the collagen synthesis stimulating activities. The collagen synthesis stimulating activities have been partially purified and shown to consist of two heat labile proteins with molecular weights of 68,000 and 6000 daltons. These factors differentially stimulate collagen synthesis in a variety of cell types but do not affect intra or extracellular collagen degrading activities. Since type IV collagen is synthesized by the myoepithelial cells of the adenocarcinomas, it is likely that the collagen synthesis stimulating factors act on the myoepithelial cells to increase their production of collagen or, they promote the conversion of stem cells in the tumor to the myoepithelial cell type.